Abstract

This specification provides a way for an author to specify, in CSS, the size, zoom factor, and orientation of the viewport that is used as the base for the initial containing block.

CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.

Status of this document

This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification.
When filing an issue, please put the text “css-device-adapt” in the title,
preferably like this:
“[css-device-adapt] …summary of comment…”.
All issues and comments are archived,
and there is also a historical archive.

1. Introduction

This section is not normative.

CSS 2.1 [CSS21] specifies an initial containing block for continuous media that has the dimensions
of the viewport. Since the viewport is generally no larger than the display,
devices with smaller displays such as phones or tablets typically present
a smaller viewport than larger devices like desktop or laptops.

Unfortunately, many documents have historically been designed against larger
viewports and exhibit a variety of bugs when viewed in smaller viewports.
These include unintended layout wrapping, clipped content, awkward scrollable
bounds, and script errors. To avoid these issues, mobile browsers generally
use a fixed initial containing block width that mimics common desktop browser
window size (typically 980-1024px). The resulting layout is then scaled down
to fit in the available screen space.

Although this approach mitigates the issues mentioned above, the downscaling
means the CSS pixel size will be smaller than recommended by CSS 2.1. Users will likely need to zoom on the content to view it
comfortably.

This mitigation is unneccessary for sites that have been designed to work
well on small viewports. This specification describes the CSS @viewport rule which allows authors to control and
opt-out of this behavior.

This specification is written from an implementation centric point of view,
making it arguably difficult to read.
Significant editorial work may be needed
to make it more understandable to different audiences.
It also should clarify which viewport is referred to by various js APIs.
See this blog post by ppk for a good discussion of these issues.

Various issues about this specification and related specifications
are listed in this report.

2. Motivating Scenarios

In this example, the document can be rendered without issue with any
size viewport. The author indicates this using the @viewport rule.

<!doctype html><html><head><title>My Site</title><style>@viewport{width:auto;}</style></head><body>
Since this document is just some simple text, it can be rendered
at any width without issue. The text will just re-wrap as needed
when viewed in a smaller viewport.
</body></html>

In this example, the document can be rendered without issue for viewports
down to 384px, but smaller sizes would clip the diagram. The author
indicates this using the @viewport rule in
combination with a media query. When the viewport would be smaller than
384px, the user agent will select 384px as the initial containing block
size and scale the resulting layout down to fit the available space.

3. Values

4. The viewport

In CSS 2.1 a viewport is a feature of a user agent for continuous media and used to
establish the initial containing block for continuous media. For paged
media, the initial containing block is based on the page area. The
page area can be set through @page rules. Hence, @viewport applies to continuous media, and @page to paged media, and they
will not interact or conflict.

This specification introduces a way of overriding the size of the viewport
provided by the user agent (UA). Because of this, we need to introduce the
difference between the initial viewport and the actual viewport.

initial viewport

This refers to the viewport before any UA or author styles have
overridden the viewport given by the window or viewing area of the UA.
Note that the initial viewport size will change with the
size of the window or viewing area.

actual viewport

This is the viewport you get after the cascaded viewport descriptors,
and the following constraining procedure have been applied.

When the actual viewport cannot fit inside the window or
viewing area, either because the actual viewport is
larger than the initial viewport or the zoom factor
causes only parts of the actual viewport to be visible,
the UA should offer a scrolling or panning mechanism.

It is recommended that initially the upper-left corners of the actual viewport and the window or viewing area are aligned if the
base direction of the document is ltr. Similarly, that the upper-right
corners are aligned when the base direction is rtl. The base direction
for a document is defined as the computed value of the direction property for the first <BODY> element of
an HTML or XHTML document. For other document types, it is the
computed direction for the root element.

"dbaron: The question is, what does this do on the desktop
browser? (And what’s a desktop browser)". Need to say that a "desktop"
browser typically have no UA styles, as opposed to the UA stylesheet outlined for current mobile
behaviour, and that no UA styles for @viewport will give "desktop"
behaviour per default (actual viewport is initial viewport).

5. The @viewport rule

UA vendors implementing this specification
are strongly encouraged to do so both for their mobile and desktop browsers.
The @viewport mechanism is designed to be usable and useful
on all browsers, not only mobile ones.
However, if support is only available on mobile browsers for a significant time,
there is a risk that authors would write @viewport rules that work on mobile
but do the wrong if applied by a desktop browser.
This would make it difficult to later add support for @viewport in desktop browsers.

An example of such misguided use would be to write @viewport{width:320px;} instead of @viewport{width: auto;} to make a document “mobile friendly”.

The @viewportat-rule consists of the @-keyword followed by a block of descriptors
describing the viewport.

The descriptors inside an @viewport rule are per document and
there is no inheritance involved. Hence declarations using the inherit keyword will be dropped. They work similarly
to @page descriptors and follow the cascading order of CSS. Hence,
descriptors in @viewport rules will override descriptors from
preceding rules. The declarations allow !important which will affect
cascading of descriptors accordingly.

This example sets the viewport to at least 320px, but otherwise match
window width if it is wider than 320px. Note that it is enough to set
the width as the height will be resolved from the width when auto.

6. Viewport descriptors

This section presents the descriptors that are allowed inside an @viewport rule. Other descriptors than those listed here will be
dropped.

Relative length values are resolved against initial values. For
instance 'em’s are resolved against the initial value of the font-size property. Viewport lengths (vw, vh, vmin, vmax) are relative to the initial viewport.

The user-agent stylesheets recommended in the informative section don’t adequately represent current implementation behaviors. Should there be a more explicit mechanism for switching between UA default behavior and requesting the CSS pixel?

The min-height and max-height descriptors are inputs to
the constraining procedure.
The height will initially be set as close as possible to the ''initial
viewport'' height within the min/max constraints.

This is a shorthand descriptor for setting both min-height and max-height.
One <viewport-length> value will set both min-height and max-height
to that value. Two <viewport-length> values will set min-height to
the first and max-height to the second.

Specifies the initial zoom factor for the window or viewing area. This
is a magnifying glass type of zoom. Interactively changing the zoom
factor from the initial zoom factor does not affect the size of the
initial or the actual viewport.

Values have the following meanings:

auto

The zoom factor is UA-dependent. The UA may use the size of the area
of the canvas on which the document is rendered to find that initial
zoom factor. See this section for a
proposed way of handling auto values for zoom.

A non-negative percentage value used as a zoom factor. A factor of
100% means that no zooming is done. Values larger than 100% gives a
zoomed-in effect and values smaller than 100% a zoomed-out effect.

Specifies the smallest allowed zoom factor. It is used as input to the constraining procedure to constrain
non-autozoom values, but also to limit the allowed zoom factor
that can be set through user interaction. The UA should also use this
value as a constraint when choosing an actual zoom factor when the
used value of zoom is auto.

Values have the following meanings:

auto

The lower limit on zoom factor is UA dependant. There will be no minimum
value constraint on the zoom descriptor used in
the constraining
procedure

Specifies the largest allowed zoom factor. It is used as input to the constraining procedure to constrain
non-autozoom values, but also to limit the allowed zoom factor
that can be set through user interaction. The UA may choose to ignore
this limit for accessbility/usability reasons – see the relevant note in
the user-zoom section. The UA should also use this
value as a constraint when choosing an actual zoom factor when the
used value of zoom is auto.

Values have the following meanings:

auto

The upper limit on zoom factor is UA dependant. There will be
no maximum value constraint on the zoom descriptor used in
the constraining
procedure

Specifies if the zoom factor can be changed by user interaction or not.

Values have the following meanings:

zoom

The user can interactively change the zoom factor.

fixed

The user cannot interactively change the zoom factor.

Authors should not suppress (with user-zoom: fixed)
or limit (with max-zoom) the ability of users to resize
a document, as this causes accessibility and usability issues.

There may be specific use cases where preventing users from zooming
may be appropriate, such as map applications – where custom zoom
functionality is handled via scripting. However, in general this
practice should be avoided.

Most user agents now allow users to always zoom, regardless
of any restrictions specified by web content – either by default, or
as a setting/option (which may however not be immediately apparent
to users).

This descriptor is used to request that a document is displayed in portrait
or landscape mode. For a UA/device where the orientation is changed upon
tilting the device, an author can use this descriptor to inhibit the
orientation change. The descriptor should be respected for
standalone web applications, and when the document is displayed
in fullscreen. It is recommended that it is ignored for normal
web navigation to avoid confusing the user.

Values have the following meanings:

auto

The UA automatically chooses the orientation based on the device’s
normal mode of operation. The UA may choose to change the orientation
of the presentation when the device is tilted.

portrait

The document should be locked to portrait presentation.

landscape

The document should be locked to landscape presentation.

7. Constraining viewport descriptor values

7.1. Definitions

For the procedure below:

Descriptors refer to the values resolved/constrained to at that point in
the procedure. They are initially resolved to their computed values.

width and height refer
to the resolved viewport size and not the shorthand descriptors. They
are both initially auto.

MIN/MAX computations where one of the arguments is auto resolve to the other argument. For instance, MIN(0.25, auto) = 0.25, and MAX(5, auto) = 5.

initial-width is the width of the initial viewport in pixels at zoom factor 1.0.

initial-height is the height of the initial viewport in pixels at zoom factor 1.0.

7.2. The procedure

The used values are resolved from the computed values going through
the steps below.

User agents are expected, but not required, to re-run this procedure
and re-layout the document, if necessary, in response to changes
in the user environment, for example if the device is tilted from
landscape to portrait mode or the window that forms the ''initial
viewport'' is resized.

However, Media Queries and Device Adaption are tethered specifications. As a result,
UAs that also
support Media Queries must re-run this procedure and re-layout the document in all cases
where changes in the user environment would cause them to re-evaluate
Media Queries.

Resolve min-zoom and max-zoom values

If min-zoom is not auto and max-zoom is not auto, set max-zoom = MAX(min-zoom,
max-zoom)

Resolve width value

Resolve height value

If height is auto,
set height = width * (initial-height /
initial-width), or height = initial-height if initial-width is 0.

8. Media Queries

For several media features, the size of the initial containing block and
the orientation of the device affects the result of a media query
evaluation, which means that the effect of @viewport rules on
media queries needs extra attention.

“To avoid circular dependencies, it is never necessary
to apply the style sheet in order to evaluate expressions. For example,
the aspect ratio of a printed document may be influenced by a style sheet,
but expressions involving device-aspect-ratio will be based
on the default aspect ratio of the user agent.”

The UA must however cascade @viewport rules separately with the initial viewport size used for evaluating media feature
expressions and other values that depend on the viewport size to avoid
circular dependencies, but use the actual viewport size when cascading all other rules.

Procedure for applying CSS rules:

Cascade all @viewport rules using the initial viewport size for values and evaluations which rely on viewport size

Compute the actual viewport from the cascaded viewport
descriptors

Cascade all other rules using the actual viewport size

The rationale for using the viewport descriptors obtained from applying
the @viewport rules for evaluating media queries for style
rules, is that media queries should match the actual viewport that the document will be layed out in and not the initial or the
one specified in the UA stylesheet. Consider the example below
given that the UA stylesheet has a viewport width of 980px, but an initial viewport width of 320px. The author has made separate
styles to make the document look good for initial containing block
widths above or below 400px. The actual viewport used will be
320px wide, and in order to match the styles with the ''actual
viewport'' width, the viewport resulting from applying the @viewport rules should be used to evaluate the media queries.

Given an initial viewport width of 320px and a UA stylesheet viewport
width of 980px, the first media query will not match, but the second will.

Below is an example where an @viewport rule relies on a media
query affected by the viewport descriptors.

The green color should be applied to a div because the initial viewport width is used to evaluate the media query
for the second @viewport rule, but the actual viewport is used for evaluating the media query when applying style rules.

It is recommended that authors do not write @viewport rules that
rely on media queries whose evaluation is affected by viewport
descriptors. Is is also recommended that the @viewport rule(s) is
placed as early in the document as possible to avoid unnecessary
re-evaluation of media queries or reflows.

9. CSSOM

The @viewport rule is exposed to the CSSOM through a new CSSRule
interface.

9.1. Interface CSSRule

The following rule type is added to the CSSRule interface. It provides identification for the new viewport rule.

This attribute represents the viewport descriptors associated
with this @viewport rule.

10. Viewport <META> element

This section is not normative.

This section describes a mapping from the content attribute of the
viewport <META> element, first
implemented by Apple in the iPhone Safari browser, to the descriptors
of the @viewport rule described in this
specification.

In order to match the Safari implementation, the following parsing
algorithm and translation rules rely on the UA stylesheet below. See the
section on UA stylesheets for an elaborate
description.

Note that these values might not fit well with all UAs. For
instance, with a min-zoom of 0.25 you will be able to fit the whole width
of the document inside the window for widths up to 1280px on a 320px wide
device like the original iPhone, but only 960px if you have a 240px
display (all widths being given in CSS pixel units).

10.1. Properties

The recognized properties in the viewport <META> element are:

width

height

initial-scale

minimum-scale

maximum-scale

user-scalable

10.2. Parsing algorithm

Below is an algorithm for parsing the content attribute of the <META> tag produced
from testing Safari on the iPhone. The testing was
done on an iPod touch running iPhone OS 4. The UA string of the
browser: "Mozilla/5.0 (iPod; U; CPU iPhone OS 4_0 like Mac OS X;
en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5
Mobile/8A293 Safari/6531.22.7". The pseudo code notation
used is based on the notation used in [Algorithms].

The whitespace class contains the following characters (ascii):

Horizontal tab (0x09)

Line feed (0x0a)

Carriage return (0x0d)

Space (0x20)

The recognized separator between property/value pairs is comma for the
Safari implementation. Some implementations have supported both commas
and semicolons. Because of that, existing content use semicolons instead
of commas. Authors should be using comma in order to ensure content works
as expected in all UAs, but implementors may add support for both to ensure
interoperability for existing content.

The separator class contains the following characters (ascii), with
comma as the preferred separator and semicolon as optional:

If a prefix of property-value can be converted to a number using strtod, the
value will be that number. The remainder of the string is
ignored.

If the value can not be converted to a number as described above,
the whole property-value string
will be matched with the following strings
case-insensitively: yes, no, device-width, device-height

If the string did not match any of the known strings, the
value is unknown.

10.3. extend-to-zoom

In order to be able to implement the functionality from <META> viewport where the viewport width
or height is extended to fill the viewing area at a given zoom level,
we introduce a UA internal value to the list of <viewport-length> values called extend-to-zoom. It will be used in width and height
declarations in the translation outlined in the section below.

This new value is necessary in order to implement the mapping for two
reasons. First, whether resolving the width/height needs to extend the
pixel length to the visible width/height for a given zoom factor depends
on the current initial width/height. <meta name="viewport" content="width=400, initial-scale=1"> yields a width of 400px for an initial-width of 320px, and 640px for an
initial width of 640px. This can not be expressed as normative min/max
descriptors that would constrain correctly when the initial width changes
like for an orientation change.

Secondly, the extended width/height also relies on cascading viewport
properties from different sources, including min-zoom and max-zoom from the UA stylesheet. For instance, if the UA stylesheet
has max-zoom: 5, and the initial width is 320px, <meta name="viewport" content="width=10"> will resolve to 64px.

Resolving 'extend-to-zoom'

The 'extend-to-zoom'
value is resolved to pixel or auto lengths as part of step 3 of the constraining procedure. Since this
is a non-normative descriptor value, the resolution is described
here. Note that max-descriptors need to be resolved to pixel lengths before min-descriptors when
'extend-to-zoom'
is a valid value.

The Viewport <META> element is placed
in the cascade as if it was a <STYLE> element,
in the exact same place in the dom, that only contains a single @viewport rule.

Each of the property/value pair from the parsing in the
previous section are translated, and added to that single at-rule
as follows:

Unknown properties

Unknown properties are dropped.

The width and height properties

The width and height viewport <META> properties are
translated into width and height descriptors, setting the min-width/min-height value to extend-to-zoom and the max-width/max-height value to the length from the
viewport <META> property as follows:

Non-negative number values are translated to pixel lengths, clamped to
the range: [1px, 10000px]

Negative number values are dropped

device-width and device-height translate to 100vw and 100vh respectively

Other keywords and unknown values translate to 1px

Some existing UA implementations use device dimensions in CSS
pixels, and some use the window dimensions (CSS pixels) for device-width /
device-height. Above, we translate to 100vw / 100vh which are the window
dimensions. The rationale is that the device dimensions would not be what
the author intended for UAs where the window is resizable or does not fill
the screen of the device.

The initial-scale, minimum-scale, and maximum-scale properties

The properties are translated into
'zoom',
'min-zoom',
and 'max-zoom'
respectively with the following translations of values.

Non-negative number values are translated to <number> values,
clamped to the range [0.1, 10]

Negative number values are dropped

yes is translated to 1

device-width and device-height are translated to 10

no and unknown values are
translated to 0.1

For a viewport <META> element that translates
into an @viewport rule with no max-zoom declaration and a
non-auto min-zoom value that is larger than the max-zoom value of
the UA stylesheet, the min-zoom declaration value is clamped to the
UA stylesheet max-zoom value.

The user-scalable property

The user-scalable property is translated into user-zoom with the following value translations.

This section presents one way of picking an actual value for the zoom descriptor when the used value is auto.

Given an initial viewport with size (initial-width,
initial-height), and a finite region within the canvas where the formatting structure is rendered (rendered-width,
rendered-height). That region is at least as large as the actual viewport.

Then, if the used value of zoom is auto, let the actual zoom factor be:

The actual zoom factor should also be further limited by the
[min-zoom, max-zoom] range.

12. UA stylesheets

This section is informative

Traditional user agents, used mostly on desktop and laptop computers, can
easily be resized to fit most websites inside the initial viewport without
breaking the layout. Using the recommendations below, sites not adding any @viewport rules themselves will continue to look and function like
they always have.

12.1. Large screen UA styles

If a user agent has an initial viewport size large enough to fit common documents without breaking the layout,
or which can easily to resized to do so,
the recommendation is to have no UA styles.
That means that it will have all descriptors initially set to auto,
and behave as it would have without support for viewport descriptors.

If a user agent supports changing orientation,
and the landscape mode’s size fits common documents as described above
but the portrait mode’s size does not,
it is recommended to set a minimum layout width
equal to that of the width in landscape mode.

@viewport {
min-width: 1024px;
}

12.2. Small screen UA styles

For smaller screen UAs, the UA could set the minimum viewport width to
typically used as an initial viewport width of a traditional user
agent (as described above).

@viewport {
min-width: 980px;
}

It is recommended that limitations in zooming capabilities are not
reflected in the UA styles but rather only affect the used values for
zoom. The min-zoom/max-zoom UA styles mentioned in
the Viewport META section are there to
give an accurate description of how the Safari browser behaves, not as
part of a recommended UA stylesheet.

Conformance

Document conventions

Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.

All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class="example",
like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the
normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification
is defined for three conformance classes:

A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.

A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid
(and ignore as appropriate)
any at-rules, properties, property values, keywords, and other syntactic constructs
for which they have no usable level of support.
In particular, user agents must not selectively ignore
unsupported property values and honor supported values in a single multi-value property declaration:
if any value is considered invalid (as unsupported values must be),
CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage,
implementers should release an unprefixed implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.

IDL Index

Issues Index

This specification is written from an implementation centric point of view,
making it arguably difficult to read.
Significant editorial work may be needed
to make it more understandable to different audiences.
It also should clarify which viewport is referred to by various js APIs.
See this blog post by ppk for a good discussion of these issues. ↵

Various issues about this specification and related specifications
are listed in this report. ↵

"dbaron: The question is, what does this do on the desktop
browser? (And what’s a desktop browser)". Need to say that a "desktop"
browser typically have no UA styles, as opposed to the UA stylesheet outlined for current mobile
behaviour, and that no UA styles for @viewport will give "desktop"
behaviour per default (actual viewport is initial viewport). ↵

min- and max- functionality can be achieved with media queries, should these be removed? ↵

The user-agent stylesheets recommended in the informative section don’t adequately represent current implementation behaviors. Should there be a more explicit mechanism for switching between UA default behavior and requesting the CSS pixel? ↵